Recent progress in optical designs and optics manufacturing technologies has enabled zoom lenses to be downsized as mentioned below despite their aperture ratios as small as approximately 2.9.
One of such state-of-the-art zoom lenses is that which includes the first or foremost lens group G1 of positive refractive power, the second or second foremost lens group G2 of negative refractive power, the third lens group G3 of positive or negative refractive power, and the fourth or rearmost lens group G4 of positive refractive power, all arranged in this sequence on the closest to an object first basis, and that varies the magnification power by varying clearances between the first lens group G1 and the second lens group G2, the second lens group G2 and the third lens group G3, and the third lens group G3 and the fourth lens group G4. In such a zoom lens, the second lens group G2 includes the first or foremost negative power lens component L21 with its hind surface shaped in concave to enhance negative power, the second or second foremost negative power lens component L22 with its front surface shaped in concave to enhance negative power, the third positive power lens component L23 that is a cemented doublet, with its front surface shaped in convex, comprised of a positive lens member L2p and a negative lens member L2n, and the fourth or rearmost negative lens component L24 with its front surface shaped in concave, all arranged in this sequence from the closest to the object to the closest to the imaging field, and the fourth lens group G4 includes a positive power lens component L41 of positive refractive power and a negative power lens component L42 that has a lens member Lasp having at least one surface shaped in asphere where the aspheric surface of the lens member Lasp in the negative power lens component L42 of the fourth lens group G4 can be expressed as in the following formula:S(y)=(y2/R)/[1+(1−k·y2/R2)1/2]+C3·|y|3+C4·y4+C5·|y|5+C6·y6+C7·|y|7+C8·y8+C10·y10+C12·y12+C14·y14where S(y) is a distance along the optical axis from a plane tangential with the vertex of any aspheric surface at height y in vertical directions from the optical axis, R is a paraxial radius of curvature, k is a conic constant, and Cn is an aspheric coefficient of the n-th order aspheric deformation. Now assuming that the aspheric coefficient of the 3rd order aspheric deformation regarding the above-mentioned aspheric surface is C3b, the zoom lens meets the requirements as defined by the formulae as follows:1×10−7≦|C3b|≦1×10−3 (See Patent Document 1 as listed below.)
Another of the prior art zoom lenses is that which includes the foremost or first lens group of positive refractive power, the second or second foremost lens group of negative refractive power, the third lens group of positive refractive power, and the rearmost or fourth lens group of positive refractive power, all arranged in this sequence on the closest to an object first basis and that permits the magnification power to vary from the wide-angle end to the telephoto end by making the first and second lens groups separate farther, the second and third lens groups come closer, and the third and fourth lens groups come closer while all the lens groups shift toward the object, although, for the focusing, the second lens groups alone is moved. Such a zoom lens meets the requirements as in the following formulae (1) to (4):0.18<|f2|/fT<0.24  (1)1.1<f1/fT<1.5  (2)0.6<f4/fT<0.9  (3)0.57<Z2/Z<0.67  (4)where fT is a focal length of the entire optics at the telephoto end, f2 is the focal length of the second lens group, f1 is the focal length of the first lens group, f4 is the focal length of the fourth lens group, Z2 equals to β2T/β2W where β2W is a magnification power of the second lens group at the wide-angle end (β2W<0), and β2T is the magnification power of the second lens group at the telephoto end (β2T<0), and Z equals to fT/fW where fW is a focal length of the entire optics at the wide-angle end (See Patent Document 2 listed below).
Further another of the prior art zoom lenses is that which includes the foremost or first lens group of positive refractive power, the second or second foremost lens group of negative refractive power, the third lens group of positive refractive power, and the rearmost or fourth lens group of positive refractive power, all arranged in this sequence from the closest to an object to the farthest thereto, and that permits the magnification power to vary from the wide-angle end to the telephoto end by making the first and second lens groups separate farther, the second and third lens groups come closer, and the third and fourth lens groups come closer. The third lens group has a leading subgroup of positive refractive power and a trailing subgroup of negative refractive power arranged in this sequence, and the trailing subgroup alone is moved in directions orthogonal to the optical axis to compensate for image blur when a camera/lens is vibrated due to hands' tremor. The fourth lens group has a concave lens piece in a position the closest to the object. (See Patent Document 3 listed below.)